One of the most important issues associated with wireless communications is how to improve the capacity of wireless multi-cell communication systems that facilitate the communications. One new area being explored is the use of directional beam antennas to improve the link margin of the forward and reverse links between base stations and wireless transmit/receive units (WTRUs). The increased gain of a directional antenna over a conventional omni-directional antenna increases received signal gain at the WTRU and the base station.
When a WTRU is powered on, the WTRU performs an initial cell search procedure, during which the WTRU measures the quality of signals received from a plurality of cells and selects one of the cells having the strongest signal level. For example, in a code division multiple access (CDMA) system, the WTRU acquires a pilot signal from a cell, and then synchronizes to that cell. In order to acquire the pilot signal, the WTRU searches all possibilities of the known pilot code and selects a cell that has the strongest pilot signal. After a cell is selected and as the WTRU moves from one location to another, the WTRU may find a better cell, (i.e., a cell with better quality measurements), to register with. The procedure of re-evaluating the selected cell is called cell re-selection or idle handoff.
A switched beam system is a system where a number of fixed directional beams are defined and a transceiver selects the directional beam that provides the greatest signal quality and the least interference. The use of a directional antenna at a WTRU requires signal level measurements on each of the directional beams in order to select the best directional mode of the antenna. The WTRU must continually monitor the received signal level in each of the directional modes and periodically reselect the best directional mode to accommodate environment changes and movement of the WTRU. The signal level received at the WTRU constantly changes due to multi-path propagation.
In the initial cell selection and cell reselection procedures, the WTRU may take advantage of the switched beam system. In designing cell selection and reselection procedures for a WTRU equipped with a switched beam antenna, there is a trade off between the time spent to perform the selection and reselection procedures and the performance of the selected or reselected cell/beam.